This invention relates generally to Night Vision Devices, and more specifically to the placement of an optical filter or similarly functional device in relation to the Night Vision Device and to the device user's eye.
Night Vision Devices (NVDs) refer to a group of electronic devices used to augment the vision of the user in low light and/or dark conditions. There are at least four classes of such devices distinguished by the type of illumination amplified by their respective electronics: image intensifiers, near infrared imaging, thermal imaging and combinations of these called fusion devices. Other types of devices may include infrared sources to illuminate the scene.
These NVDs utilize a display to convert the electronic signals from the detectors into an image visible by the human eye. Depending on the type of ENVD, the image display may be an analog phosphor typified by the NVD AN/PVS-14 Image Intensifier P-43 phosphor or a digital display in black & white or color as typified by the RECON III or a display that optically combines the two display types typified by the ENVG.
It is a well-known fact that such displays are sufficiently bright to cause the eye to function in the photopic regime or the mesopic intermediate regime between photopic and scotopic. The human eye has two modes of function depending on the incident illumination levels (e.g. Duplicity Theory as discussed in Graham (Editor) Vision and Visual Perception, John Wiley and Sons, 1966). In this regard, the term “photopic” refers to the eye function at a relatively high level of illumination (daylight) and “scotopic” refers to function at relatively low light levels (night). Acuity and color sensitivity are significantly better for the photopic regime; sensitivity to low illumination is significantly better with scotopic vision.
An important factor is the time period required to transition from photopic to scotopic vision, a phenomenon termed dark adaptation time. Depending on prior conditions, this dark adaptation time can be 45 minutes or longer. During this transition, the eye has difficulty detecting objects resulting in a condition called night blindness. Because the NVD display is bright, therefore when the user of the NVD removes the device or looks away from the display, he/she endures night blindness during the dark adaptation time.
Special filters such as that disclosed in Kreutzig U.S. Pat. No. 6,158,865 have been designed to shorten the dark adaptation time and reduce this night blindness.
It is important that any optical filter employed to reduce the facial signature function to maintain the user's peripheral vision, to reduce the dark adaptation time, and yet work properly with all types of image displays.
The displays of the NVDs produce a general illumination of the user's face and/or eye socket. Termed facial signature or splash, this is very undesirable because it will disclose the presence and location of the NVD user to hostile persons who happen to be looking in the direction of the NVD user. As illustrated in FIG. 1, the NVD 2 is utilized by the wearer 1 by holding the NVD up to the eye and viewing through the attached eyepiece 3 but not touching the facial surface surrounding the eye. This existing design results in a portion of the face 4 being illuminated by the display of the NVD which is undesirable.
Attempts to reduce this facial signature by placement of standard accordion rubber eyecups on the eyepiece that interface tightly to the user's eye essentially eliminate the user's peripheral vision. This result significantly and detrimentally compromises the user's situational awareness when using an NVD with such an eyecup. In FIG. 2, a flexible eyecup 5 made of pleated rubber is frequently attached to the eyepiece 3 in order to block this illumination of the face. Note that because the flexible eyecup 5 touches the face, the peripheral vision of the use is significantly reduced. This is undesirable.
Some previous designs have placed filters over the final optical element of the eyepiece. In FIG. 3, an optical filter 6 is attached to the eyepiece 3 which reduces the illumination 7 on the user's face. These filters are described in Kreutzig patent U.S. Pat. No. 8,226,251.
While this succeeds in a major objective as described above, placement of the filter in this location subjects the filter to scratching, difficulty of field installation and outright loss of the filter.